The answer is D,
cuz if you equate mw^2r to Bqv,
according to the question, q=e and m=m obv,
Using v=wr we can cancel v on both sides.
So, mw=Be
then using w=2pi/t
we get 2pim/T = Be
rearrange that to make T the subject and there you go!

(Original post by handsome7654)
The answer is D,
cuz if you equate mw^2r to Bqv,
according to the question, q=e and m=m obv,
Using v=wr we can cancel v on both sides.
So, mw=Be
then using w=2pi/t
we get 2pim/T = Be
rearrange that to make T the subject and there you go!

that's what i did and i got D but my mark scheme says the answer is B...

(Original post by jones_wise)
I've got that question in a booklet made by my college, and the answer in their mark scheme is D. I agree with handsome7654's working, so it must just be a mistake in the mark scheme.

The diagram shows two charges, +4 μC and –16 μC, 120 mm apart. What is the distance from the +4 μC
charge to the point between the two charges, where the resultant electric potential is zero?
A) 24mm
B) 40mm
C) 80mm
D) 96mm

The diagram shows two charges, +4 μC and –16 μC, 120 mm apart. What is the distance from the +4 μC
charge to the point between the two charges, where the resultant electric potential is zero?
A) 24mm
B) 40mm
C) 80mm
D) 96mm

Thanks

you use the electric potential formula

V=Q/4piepsilon0 x r but the 4piepsilon0 cancels for both so jus ignore it

so for the 4uC you do V=4uC/120 and for the -16uC you do V=-16uC/120-x

(Original post by soldiersixteen)
Thanks.
When i equate them, I get
480u-4ux= -1920u
-4ux=-2400u
x=600

where am i messing up?

Do it like this;
Cuz the sum of the potentials at a distance x away from 4uC is 0, you get
+4u/4piepsilon0(x) - 6u/4piepsilon0(20-x) = 0, then add 6u/4piepsilon0(12-x) to both sides,
4u/x = 6u/120-x (the 4piepsilon0 bit cancels), then multiply by (120-x)
480u - 4ux = 16ux
so x = 480u/20u = 24mm!

I have just spent all day printing off every multi choice question from the past 10 years. Highligting the right answer and cutting them out. Then placing them in 7 piles (pile for each topic). After removing recent questions and questions that have already been repeated once I think I can guess a few of out questions, well the ones they will be re-using anyway.

(Original post by maths134)
I have just spent all day printing off every multi choice question from the past 10 years. Highligting the right answer and cutting them out. Then placing them in 7 piles (pile for each topic). After removing recent questions and questions that have already been repeated once I think I can guess a few of out questions, well the ones they will be re-using anyway.

(Original post by maths134)
I have just spent all day printing off every multi choice question from the past 10 years. Highligting the right answer and cutting them out. Then placing them in 7 piles (pile for each topic). After removing recent questions and questions that have already been repeated once I think I can guess a few of out questions, well the ones they will be re-using anyway.

Can I ask how you got access to all those past papers? If there is a website for them, can u give me the link plzz?? Really need some practice cuz iv only been getting C/Ds on my practice papers!!